The daily average high (red line) and low (blue line) temperature, with 25th to 75th and 10th to 90th percentile bands. The thin dotted lines are the corresponding average perceived temperatures.

The figure below shows you a compact characterization of the hourly average temperatures for the quarter of the year centered on May. The horizontal axis is the day, the vertical axis is the hour of the day, and the color is the average temperature for that hour and day.

Clouds

The month of May in Blackhawk experiences decreasing cloud cover, with the percentage of time that the sky is overcast or mostly cloudy decreasing from 38% to 27%.

The clearest day of the month is May 31, with clear, mostly clear, or partly cloudy conditions 73% of the time.

For reference, on January 11, the cloudiest day of the year, the chance of overcast or mostly cloudy conditions is 57%, while on July 20, the clearest day of the year, the chance of clear, mostly clear, or partly cloudy skies is 92%.

Precipitation

A wet day is one with at least 0.04 inches of liquid or liquid-equivalent precipitation. In Blackhawk, the chance of a wet day over the course of May is decreasing, starting the month at 9% and ending it at 4%.

For reference, the year's highest daily chance of a wet day is 29% on February 20, and its lowest chance is 0% on July 24.

The percentage of days in which various types of precipitation are observed, excluding trace quantities: rain alone, snow alone, and mixed (both rain and snow fell in the same day).

Rainfall

To show variation within the month and not just the monthly total, we show the rainfall accumulated over a sliding 31-day period centered around each day.

The average sliding 31-day rainfall during May in Blackhawk is gradually decreasing, starting the month at 0.7 inches, when it rarely exceeds 1.8 inches, and ending the month at 0.3 inches, when it rarely exceeds 1.0 inches.

The average rainfall (solid line) accumulated over the course of a sliding 31-day period centered on the day in question, with 25th to 75th and 10th to 90th percentile bands. The thin dotted line is the corresponding average liquid-equivalent snowfall.

Sun

Over the course of May in Blackhawk, the length of the day is increasing. From the start to the end of the month, the length of the day increases by 49 minutes, implying an average daily increase of 1 minute, 38 seconds, and weekly increase of 11 minutes, 29 seconds.

The shortest day of the month is May 1, with 13 hours, 47 minutes of daylight and the longest day is May 31, with 14 hours, 36 minutes of daylight.

The number of hours during which the Sun is visible (black line). From bottom (most yellow) to top (most gray), the color bands indicate: full daylight, twilight (civil, nautical, and astronomical), and full night.

The latest sunrise of the month in Blackhawk is 6:11 AM on May 1 and the earliest sunrise is 24 minutes earlier at 5:47 AM on May 31.

The earliest sunset is 7:58 PM on May 1 and the latest sunset is 25 minutes later at 8:23 PM on May 31.

Daylight saving time is observed in Blackhawk during 2018, but it neither starts nor ends during May, so the entire month is in daylight saving time.

For reference, on June 21, the longest day of the year, the Sun rises at 5:45 AM and sets 14 hours, 47 minutes later, at 8:33 PM, while on December 21, the shortest day of the year, it rises at 7:19 AM and sets 9 hours, 33 minutes later, at 4:52 PM.

The solar day over the course of May. From bottom to top, the black lines are the previous solar midnight, sunrise, solar noon, sunset, and the next solar midnight. The day, twilights (civil, nautical, and astronomical), and night are indicated by the color bands from yellow to gray.

Humidity

We base the humidity comfort level on the dew point, as it determines whether perspiration will evaporate from the skin, thereby cooling the body. Lower dew points feel drier and higher dew points feel more humid. Unlike temperature, which typically varies significantly between night and day, dew point tends to change more slowly, so while the temperature may drop at night, a muggy day is typically followed by a muggy night.

The chance that a given day will be muggy in Blackhawk is essentially constant during May, remaining around 0% throughout.

For reference, on September 10, the muggiest day of the year, there are muggy conditions 0% of the time, while on January 1, the least muggy day of the year, there are muggy conditions 0% of the time.

Wind

This section discusses the wide-area hourly average wind vector (speed and direction) at 10 meters above the ground. The wind experienced at any given location is highly dependent on local topography and other factors, and instantaneous wind speed and direction vary more widely than hourly averages.

The average hourly wind speed in Blackhawk is increasing during May, increasing from 8.8 miles per hour to 9.9 miles per hour over the course of the month.

For reference, on June 22, the windiest day of the year, the daily average wind speed is 10.2 miles per hour, while on January 21, the calmest day of the year, the daily average wind speed is 6.4 miles per hour.

The percentage of hours in which the mean wind direction is from each of the four cardinal wind directions (north, east, south, and west), excluding hours in which the mean wind speed is less than 1 mph. The lightly tinted areas at the boundaries are the percentage of hours spent in the implied intermediate directions (northeast, southeast, southwest, and northwest).

Water Temperature

Blackhawk is located near a large body of water (e.g., ocean, sea, or large lake). This section reports on the wide-area average surface temperature of that water.

The average surface water temperature in Blackhawk is essentially constant during May, remaining around 53°F throughout.

The lowest average surface water temperature during May is 53°F on May 6.

The daily average water temperature (purple line), with 25th to 75th and 10th to 90th percentile bands.

Growing Season

Definitions of the growing season vary throughout the world, but for the purposes of this report, we define it as the longest continuous period of non-freezing temperatures (≥ 32°F) in the year (the calendar year in the Northern Hemisphere, or from July 1 until June 30 in the Southern Hemisphere).

The growing season in Blackhawk typically lasts for 10 months (305 days), from around February 4 to around December 7, rarely starting after March 12, or ending before November 15.

The month of May in Blackhawk is reliably fully within the growing season.

Growing degree days are a measure of yearly heat accumulation used to predict plant and animal development, and defined as the integral of warmth above a base temperature, discarding any excess above a maximum temperature. In this report, we use a base of 50°F and a cap of 86°F.

The average accumulated growing degree days in Blackhawk are increasing during May, increasing by 297°F, from 437°F to 734°F, over the course of the month.

The average growing degree days accumulated over the course of May, with 25th to 75th and 10th to 90th percentile bands.

Solar Energy

This section discusses the total daily incident shortwave solar energy reaching the surface of the ground over a wide area, taking full account of seasonal variations in the length of the day, the elevation of the Sun above the horizon, and absorption by clouds and other atmospheric constituents. Shortwave radiation includes visible light and ultraviolet radiation.

The average daily incident shortwave solar energy in Blackhawk is gradually increasing during May, rising by 1.0 kWh, from 7.0 kWh to 7.9 kWh, over the course of the month.

The average daily shortwave solar energy reaching the ground per square meter (orange line), with 25th to 75th and 10th to 90th percentile bands.

Topography

For the purposes of this report, the geographical coordinates of Blackhawk are 37.821 deg latitude, -121.908 deg longitude, and 899 ft elevation.

The topography within 2 miles of Blackhawk contains very significant variations in elevation, with a maximum elevation change of 1,473 feet and an average elevation above sea level of 1,060 feet. Within 10 miles contains very significant variations in elevation (3,704 feet). Within 50 miles also contains extreme variations in elevation (4,324 feet).

The area within 2 miles of Blackhawk is covered by grassland (55%) and shrubs (32%), within 10 miles by grassland (65%) and artificial surfaces (17%), and within 50 miles by cropland (26%) and grassland (21%).

Data Sources

This report illustrates the typical weather in Blackhawk year round, based on a statistical analysis of historical hourly weather reports and model reconstructions from January 1, 1980 to December 31, 2016.

Temperature and Dew Point

There are 3 weather stations near enough to contribute to our estimation of the temperature and dew point in Blackhawk.

The estimated value at Blackhawk is computed as the weighted average of the individual contributions from each station, with weights proportional to the inverse of the distance between Blackhawk and a given station.

Other Data

All other weather data, including cloud cover, precipitation, wind speed and direction, and solar flux, come from NASA's MERRA-2 Modern-Era Retrospective Analysis . This reanalysis combines a variety of wide-area measurements in a state-of-the-art global meteorological model to reconstruct the hourly history of weather throughout the world on a 50-kilometer grid.

Disclaimer

The information on this site is provided as is, without any assurances as to its accuracy or suitability for any purpose. Weather data is prone to errors, outages, and other defects. We assume no responsibility for any decisions made on the basis of the content presented on this site.

We draw particular cautious attention to our reliance on the MERRA-2 model-based reconstructions for a number of important data series. While having the tremendous advantages of temporal and spatial completeness, these reconstructions: (1) are based on computer models that may have model-based errors, (2) are coarsely sampled on a 50 km grid and are therefore unable to reconstruct the local variations of many microclimates, and (3) have particular difficulty with the weather in some coastal areas, especially small islands.

We further caution that our travel scores are only as good as the data that underpin them, that weather conditions at any given location and time are unpredictable and variable, and that the definition of the scores reflects a particular set of preferences that may not agree with those of any particular reader.